The present invention relates to a collet mechanism and a method of molding a cannula to a syringe barrel, specifically the collet mechanism and method of molding a fine gauge cannula to a polymeric syringe barrel to form a staked needle device.
The process of bonding a cannula to a syringe barrel is a critical step in the manufacture of syringes. The cannula must be precisely positioned and then bonded to the syringe without being damaged. The slightest bend or burr in the end of the cannula increases the perceived pain by a patient. The cannula is preferably directly inline with the center axis of the syringe. The cannula must also be sufficiently bonded to the syringe to withstand a minimum 22N pull test rating in accordance with the International Standards Organization's (ISO's) standards. For polymeric syringes, the cannula is typically interference fit into a preformed syringe barrel in a similar manner to what is done with a glass syringe. The cannula is often affixed to the syringe by an adhesive. However, it is undesirable to use an adhesive to affix a cannula to a syringe because the adhesive may contain toxic chemicals that could leach into the syringe contents, comprised of one or more drug(s)/biological compound(s), and could alter the efficacy and/or stability of the syringe contents that eventually enter the patient.
In order to avoid the use of an adhesive, the cannula (typically a 25 gauge needle or larger) is typically crimped toward its proximal end and then the syringe barrel is insert molded over the proximal end of the cannula, so that the syringe molds over the crimped proximal end to securely grip and retain the cannula to the syringe. Though crimping the proximal end may avoid the use of an adhesive, finer gauge cannulas (i.e. 27 to 33 gauge) cannot usually be crimped without potentially damaging the cannula due to their small physical dimensions. Specifically, due to the very thin wall of the tubing of such finer gauge cannulas, it is physically impossible to crimp the outside diameter of such cannulas without completely closing down the inner diameter of the cannulas. Regardless of the attachment method, it is difficult to position, hold, and/or mount a fragile, fine gauge cannula without damaging the cannula.
What is therefore needed, but not provided in the prior art, is a mechanism that holds the cannula, particularly a small diameter cannula, in place during the molding process to securely attach the cannula directly to a polymeric syringe barrel without the use of an adhesive or the need to crimp the cannula, and without damaging the cannula, specifically the sharp beveled tip of the cannula.